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TLV7031: Comparator output with pull up resistors

Tahir Mahmood
Expert 1160 points
Part Number: TLV7031

I have a pull up of 10k Ohm at the positive input pin and a pull up of 470 Ohms at - negative input pin. I am using only  +5V power supply. I am getting low ( zero volts) at the output. Is it possible that the output becomes high for these pull ups with a different TLV7031 or with a different temperature?  Is it safe to use these pull up values to always get a low at the output for 0°C to 100°C and for all the manufactured boards.

  • 0
    TI__Guru 53826 points

    Hello Tahir,

    Can you please provide a full schematic?

    Pulling both inputs up to the same voltage is an undefined condition (IN+ = IN-). The output is indeterminate.

    When both inputs are equal, the output will be determined purely by the individual devices internal offset voltage. The offset can be positive or negative, so you have a 50% chance of high, or 50% chance of low. There is no way we could "guarantee" the output would be low unless you add an offset to the input to bias the output towards low. This is true of ANY comparator.

  • 0 in reply to Paul Grohe
    Expert 1160 points

    Thanks for your reply. We have an old circuit with a relay. The switching of the circuit brings the two inputs in the pull up position for a short duration during switching. The relay is sometimes not working as desired so I am trying to find the problems in the circuit.

    When I reverse the pull up on the two pins. 10k on -ve pin and 470 Ohm on the +ve the output is reversed becomes high.

    Is it necessary to always have a pull down on the each of the +ve and -ve input pins? 

  • 0 in reply to Tahir Mahmood
    Guru 281760 points

    The TLV7031 is a CMOS comparator with very low input bias currents, so the two pull-up resistors will not have a noticeable voltage drop.

    In order to have a voltage of less than 5 V at the inverting input, you indeed have to add a pull-down resistor to create a voltage divider for this input.

  • 0 in reply to Clemens Ladisch
    TI__Genius 11701 points

    Thanks Clemens.

    Tahir,

    It may be easier to tie the inputs to two separate and different voltages so the comparator can have a defined output condition.